1. Field of the Invention
This invention relates to electron emitters for electric propulsion systems. Particularly, this invention relates to electron emitters for Hall effect thrusters in space applications.
2. Description of the Related Art
Electric propulsion systems have been operated in space applications for decades. Electric propulsion systems are well suited for space applications where their low thrust (compared to conventional chemical thrusters) can be tolerated to realize the benefits of their high efficiency. Two basic types of practical electric propulsion systems have been developed, electrostatic ion thrusters and Hall effect thrusters. In general, both types of electric thruster operate by ionizing a gas (i.e., developing a plasma) and accelerating those ions with an electric field. The accelerated ions are ejected in a beam to impart a thrust reaction. Electrostatic ion thrusters develop the accelerating electric field using spaced metal grids whereas Hall effect thrusters develop the electric field near an open end of an annular discharge chamber where the radial component of an applied magnetic field is at its maximum. The intense magnetic field traps electrons and, in order to maintain current continuity, a sharp rise in the electric field is forced to occur that accelerates ions created in this region through ionizing collisions with electrons.
Both electrostatic and Hall effect electric propulsion systems employ electron emitters to develop the plasma and neutralize the ion beam. Present electron emitters for electrostatic and Hall effect electric propulsion systems typically employ a hollow geometry with a barium-oxide impregnated insert that acts as a thermionic electron emitter.
In conventional Hall effect thrusters electron emitter cathodes are mounted external to the annular discharge chamber on one side. As thrust power is increased beyond approximately 5 kW, the ability of the cathode to uniformly distribute electrons around the circumference of the annual thruster chamber in an effective manner diminishes. This can introduce asymmetries in the developed ion beam while also impairing performance and reducing life of the thruster. Accordingly, some Hall effect thrusters employing central cathode configurations have be developed.
Since the 1960s NASA and the commercial aerospace industry have been developing, testing, and flying barium-oxide (Bao) impregnated dispenser cathodes in ion thrusters, Hall thrusters, plasma contactors, and plasma neutralizers. In addition, over 238 Russian Hall thrusters have been flown since 1971 with lanthanum hexaboride (LaB6) hollow cathodes. Further, LaB6 electron emitters have been used extensively in university research devices and industrial applications such as plasma sources, ion sources, arc melters, optical coaters, ion platers, scanning electron microscopes, and many other applications.
Lanthanum hexaboride was first developed as an electron emitter by Lafferty (Lafferty, J. M., “Boride Cathodes,” Journal of Applied Physics, Vol. 22, No. 3, March 1951, pp. 299-309) in the 1950s. The thermionic emission of lanthanumboron compounds as a function of the surface stoichiometry was extensively studied by several authors. See, Storms, E., and Mueller, B., “A Study of Surface Stoichiometry and Thermionic Emission Using LaB6, Journal of Applied Physics, Vol. 50, No. 5, May 1979, pp. 3691-3698; Storms, E., and Mueller, B., “Phase Relationship, Vaporization and Thermodynamic Properties of the Lanthanum-Boron System,” Journal of Chemical Physics, Vol. 82, No. 1, January 1978, pp. 51-59; Jacobson, D., and Storms, E. K., “Work Function Measurement of Lanthanum-Boron Compounds,” IEEE Transactions on Plasma Science, Vol. 6, No. 2, June 1978, pp. 191-199; and Pelletier, J., and Pomot, C., “Work Function of Sintered Lanthanum Hexaboride,” Applied Physics Letters, Vol. 34, No. 4, February 1979, pp. 249-251.
The major advantage for using LaB6 cathodes over conventional BaO impregnated dispenser cathodes is the robustness, high-current density and long life exhibited by LaB6 electron emitters. Lanthanum hexaboride cathodes are routinely used with all noble gases from helium to xenon, reactive gases including hydrogen and oxygen, and various other materials including liquid metals such as bismuth. Although not previously employed in space applications in the U.S., the space heritage of lanthanum hexaboride cathodes in Russian thrusters is considerable, and the industrial experience in dealing with the higher operating temperatures and materials compatibility issues is extensive.
The first flight of Russian stationary plasma thruster (SPT) Hall thrusters in 1971, and all subsequent flights, used lanthanum hexaboride cathodes. See, Kim, V., “Electric Propulsion Activity in Russia,” IEPC Paper 2001-005, 2001. The first reported use of LaB6 in the U.S. in a hollow cathode was by Goebel et al. in 1978, and the development of a high-current LaB6 cathode for plasma sources that dealt with supporting and making electrical contact with the material was described by Goebel et al. in 1985. See, Goebel, D. M., Crow, J. T., and Forrester, A. T., “Lanthanum Hexaboride Hollow Cathode for Dense Plasma Production,” Review of Scientific Instruments, Vol. 49, No. 4, April 1978, pp. 469-472; and Goebel, D. M., Hirooka, Y., and Sketchley, T., “Large Area Lanthanum Hexaboride Electron Emitter,” Review of Scientific Instruments, Vol. 56, No. 9, September 1985, pp. 1717-1722. The lanthanum-boron system can comprise combinations of stable LaB4, LaB6, and LaB9 compounds, with the surface color determined by the dominate compound. The evolution of LaB4 to LaB9 compounds is caused either by preferential sputtering of the boron or lanthanum atoms at the near surface by energetic ion bombardment, or by preferential chemical reactions with the surface atoms. Lanthanum-boride compounds, heated to in excess of 1000° C. in vacuum, evaporate their components at a rate that produces a stable LaB6.0 surface.
Conventional space hollow cathodes typically use a porous tungsten insert that is impregnated with an emissive mix of barium and calcium oxides and alumina. This configuration is called a dispenser cathode because the tungsten matrix acts as a reservoir for barium that is “dispensed” from the pores to activate the emitter surface. Chemical reactions in the pores or at the surface at high temperature evolve a barium-oxide dipole attached to an active site on the tungsten substrate, which reduces the work function of the surface to about 2.06 eV at temperatures in excess of 1000° C. Because chemistry is involved in the formation of the low work function surface, dispenser cathodes are subject to poisoning that can significantly increase the work function. Care must be taken in handling the inserts and in the vacuum conditions used during operation and storage of these cathodes to avoid poisoning by water vapor and impurities in the gas that can shorten the lifetime or even prevent cathode emission. One of the major drawbacks of using BaO dispenser cathodes in electric propulsion applications is the extremely high feed gas purity presently specified by NASA and commercial thruster manufacturers to avoid these poisoning issues, which has resulted in a special “propulsion-grade” xenon with 99.9995% purity and extensive spacecraft feed system cleaning techniques to be required.
On the other hand, Lanthanum hexaboride is a crystalline material made by press sintering LaB6 powder into rods or plates and then machining the material to the desired shape. Polycrystalline LaB6 cathodes have a work function of about 2.67 eV depending on the surface stoichiometry, and will emit over 10 A/cm2 at a temperature of 1650° C. Because the bulk material is emitting, there is no chemistry involved in producing the low work function surface and thus, LaB6 cathodes are insensitive to impurities and air exposures that would normally destroy a BaO dispenser cathode. In addition, the cathode life is determined primarily by the evaporation rate of the bulk LaB6 material at typical operating temperatures. The higher operating temperature of LaB6 and the need to support and make electrical contact with LaB6 with compatible materials has perhaps unjustly limited their use in the U.S. space program.
Near the lower end of the 5 kW to 10 kW power range in Hall thrusters, where central cathode configurations begin to become more desirable, it is especially challenging to integrate a central cathode due to volume constraints resulting from the inner magnetic circuit of the thruster. These volume limitations stress the design of the central cathode, necessitating miniaturization, which in turn makes achieving an acceptable thermal design more difficult while ensuring sufficient cathode life for a particular application.
In view of the foregoing, there is a need in the art for apparatuses and methods for efficient and effective electron emitters for electric propulsion systems, especially in space applications. In addition, there is a need for such apparatuses and methods to be compact and capable of operating at high current levels. There is also a need for such apparatuses and methods to deal with very high operating temperatures. There is particularly a need for such systems and apparatuses in Hall effect thrusters operating at higher power levels. These and other needs are met by the present invention as detailed hereafter.